Phenol is an important product in the chemical industry and is useful in, for example, the production of phenolic resins, bisphenol A, ε-caprolactam, adipic acid, and plasticizers.
Currently, the most common route for the production of phenol is the Hock process via cumene. This is a three-step process involving alkylation of benzene with propylene to produce cumene, followed by oxidation of the cumene to the corresponding hydroperoxide and then cleavage of the hydroperoxide to produce equimolar amounts of phenol and acetone. However, the world demand for phenol is growing more rapidly than that for acetone. In addition, the cost of propylene is likely to increase, due to a developing shortage of propylene. Thus, a process that does not require propylene as a feed and coproduces higher ketones, rather than acetone, may be an attractive alternative route to the production of phenol.
One such process involves the catalytic hydroalkylation of benzene to produce cyclohexylbenzene, followed by the oxidation of the cyclohexylbenzene (analogous to cumene oxidation) to cyclohexylbenzene hydroperoxide, which is then cleaved to produce phenol and cyclohexanone in substantially equimolar amounts. Such a process is described in, for example, U.S. Pat. No. 6,037,513, in which the hydroalkylation catalyst is a bifunctional catalyst comprising at least one hydrogenation metal and a molecular sieve of the MCM-22 family.
However, one problem in producing phenol via cyclohexylbenzene is that the oxidation and cleavage processes produce significant quantities of impurities, such as cyclohexenone, in addition to the desired products. Cyclohexenone is a particularly troublesome impurity since it acts substantially like cyclohexanone in the fractionation systems conventionally employed to separate the product mixtures into individual components. As a result the cyclohexenone tends to concentrate in the cyclohexanone product.
Existing methods of removing cyclohexenone from product streams containing cyclohexanone normally involve reacting the cyclohexenone with other components in the product stream, such as cyclohexanone, to produce heavies which can be removed by fractionation. However, these processes reduce the yield of the desired cyclohexanone and also potentially require additional distillation equipment.
There is therefore a need for an improved method for removing cyclohexenone from the product streams produced in the oxidation of cyclohexylbenzene and the cleavage of cyclohexylbenzene hydroperoxide. It has now been found that, because cyclohexenone has two double bonds in a conjugated system, its olefinic double bond is substantially more reactive than the single ketone double bond in cyclohexanone. This difference in reactivity is utilized, in accordance with the invention, to selectively hydrogenate cyclohexenone to cyclohexanone with minimal yield loss of the cyclohexanone.
U.S. Pat. No. 5,292,960 discloses a process for the purification of cyclohexanone which has been obtained by dehydrogenation of a cyclohexanone/cyclohexanol mixture, wherein the dehydrogenation mixture is treated in the gas or liquid phase with H2 on a hydrogenation catalyst at 20° C. to 180° C. and at an H2 pressure of 0.1 to 15 bar. The cyclohexanone/cyclohexanol mixture employed in the '960 patent is obtained either by oxidation of cyclohexane or by hydrogenation of phenol. In either event, the impurities removed in the purification process are said to be “not completely known”(column 1, lines 29 and 30).
U.S. Pat. No. 7,199,271 discloses a method for reducing the concentration of cyclohexenone in an organic mixture comprising cyclohexenone, particularly a mixture of cyclohexanol and cyclohexenone produced by oxidizing cyclohexane. The method comprises treating the organic mixture with an effective amount of additive comprising at least one of sulfurous acid, a salt of sulfurous acid, or an alkali hydroxide.